Add loaders for gl, glx, wgl generated with glad. Some issues remaining
- Do we need to include the license of glad (given that the loader is generated with the tool) - I would say likely yes (because why not?) (also note that it is not in this patch)
- Will loading wgl functions in this way work (may nullptr be passed as HDC?)
- Is there a way to get premake to compile a C lib without C++ flags? otherwise compiling the glad lib as C++ seems to work (changing extensions .c to .cpp)
Next step: remove all deprecated OpenGL usage and generate core contexts where available (so newer GL functions are available on OS X).